1. Field of the Invention
The present invention relates to a method of forming a resist pattern, and more particularly to a method of forming a resist pattern improved to provide a resist pattern of a favorable configuration with high resolution and wide depth of focus. Also, the present invention relates to an acid water-soluble material composition used in such a method.
2. Description of the Background Art
The formation process of a resist pattern is critical in the manufacturing method of a large scale integrated circuit (LSI) such as 4-M and 16-M dynamic random access memories (DRAM). The method of forming a resist pattern includes the steps of selectively irradiating a positive type photoresist of novolac resin and naphthoquinone diazide with a g-line ray (wavelength 463 nm) of a mercury lamp, followed by a developing step.
In recent years, the integration density has become higher such as the level of 16M and 64M. In a LSI having such microstructures, an i-line ray (wavelength 365 nm) which has a wavelength shorter than that of a g-line ray is used as the light source in forming a resist pattern.
As the integration density of a LSI is further increased, it will become difficult to manufacture a resist pattern that is below half micron in a stable manner using an i-line ray. There has been intensive research efforts to form a resist pattern using a KrF excimer laser (wavelength 248 nm) and an ArF excimer laser (wavelength 193 nm) which have a shorter wavelength as a light source.
In the technology using such an excimer laser beam, not only modification of the type of light source, but also development of a new resist material is essential. This is because a high resolution resist pattern that has a sidewall perpendicular to the substrate cannot be obtained if a conventional novolac-naphthoquinone diazide type resist which was used for g-line and i-line is employed as the resist subjected to excimer laser beam due to its great absorbance. A novel resist material that has superior characteristics with respect to an excimer laser beam has not yet been found. In view of such problems of the light source and the material of a resist, it is preferable to develop a technique for forming a resist pattern that provides high resolution and wide process margin from an evolution of conventional technology, i.e., using i-line and g-line rays and a general novolac-naphthoquinone diazide type resist.
A conventional method of forming a resist pattern using a novolac-naphthoquinone diazide type resist will be described hereinafter.
Referring to FIG. 7, a resist is applied on a substrate 1, which is prebaked to result in a resist film 2. Prebaking is carried out using a hot plate 8.
Referring to FIG. 8, an i-line beam 5 is selectively directed towards resist film 2 using a reticle 4. i-light beam 5 is formed by an i-line stepper. Here, resist film 2 is divided into an exposed region 6a and a non-exposed region 6b. Referring to FIG. 11, the exposed region has naphthoquinone diazide (NQD) rendered to indene carboxylic acid via indene ketene. Indene carboxylic acid is soluble in an alkali developer.
Referring to FIG. 9, baking is carried out after exposure and before developing. This baking is carried out for the purpose of reducing the effect of a standing wave that occurs due to interference between incident light and reflected light from the underlying substrate to obtain a favorable pattern configuration of high resolution. This step is called post exposure baking (PEB). PEB is carried out using hot plate 8.
Referring to FIG. 10, resist film 2 is developed using an alkali developer such as tetramethyl ammonium hydroxide solution. Exposed region 6a is removed to result in a resist pattern 7.
The above-described conventional resist pattern formation had the following problems.
Referring to FIG. 10, a resist pattern having a sidewall perpendicular to substrate 1 cannot be obtained due to the increase in the reduced film of the non-exposed portion. This is because light does not reach the bottom of resist film 2.
In order to solve the above-described problem, i.e. to suppress the film reduction of the non-exposed portion, an improvement is effected of reducing the solubility speed of the resist film with respect to the developer for the currently used high resolution positive type photoresists. However, with this improvement, the solubility speed of the exposed portion could not be increased. Sufficient difference in the solubility speed between an exposed portion and a non-exposed portion could not be achieved. Therefore, sufficient resolution and favorable resist configuration could not be obtained.